Unsaturated polyester resins have been employed commercially in various fiber reinforced fabrication systems including among others matched metal-die compression, transfer, and injection molding. These systems involve curing a formulated compound at high temperatures and pressures in hardened and chrome-plated molds. These methods provide the highest volume and highest part uniformity of any thermoset molding technique.
A technical improvement that has made a significant contribution to commercial thermosetting molding technology is the use of low profile additives to reduce shrinkage during the curing reaction, and to thereby improve dimensional stability and surface smoothness. Low profile additives are, in general, thermoplastic polymers, such as, vinyl acetate polymers, polystyrene, acrylic polymers, and polycaprolactones. There are a number of theories that seek to explain the low profile or anti-shrinkage action of these polymers, but the one that seems to best explain the phenomenon is the following:
The low profile additive is at least partly soluble in the uncured thermosetting resin such as a polyester/styrene solution. As the polyester/styrene mixture crosslinks, the thermoplastic polymer becomes incompatible or less soluble and at least partly comes out of solution. This action causes a volume expansion that compensates for the shrinkage that occurs when the polyester/styrene mixture crosslinks.
A major advance in commercial thermosetting molding technology was the introduction several years ago of chemically thickened systems. Chemical thickening is always employed in sheet molding compounds ("SMC"), and is increasingly being used in bulk molding compounds ("BMC"). In such systems, an alkaline material such as magnesium oxide or magnesium hydroxide is added to, for example, an uncured polyester along with fillers, glass fiber, and other standard materials. The alkaline material interacts with residual acidity in the polyester to build viscosity. The thickened system is relatively tackfree and easy to handle, and the high viscosity carries the glass fiber reinforcement to the extremities of the mold during crosslinking of the system. Thus, the use of thickened systems has made a major contribution to the commercial expansion of polyester molding.
While low profile unsaturated polyester fiber glass reinforced molding systems have gained wide acceptance in the transportation industry because of good surface appearance, dimensional stability, physical properties, productions and assembly costs and weight savings versus metal, there is still a need for further improvement in reducing rippling and waviness in surface sensitive areas that are molded down a "draw" area of the mold.
U.S. Pat. No. 3,997,627 (Ichimura et al.) describes an unsaturated polyester composition for molding compounds consisting essentially of (1) 20 to 70 parts by weight of an unsaturated polyester obtained by condensing an acid component comprising a major amount of an alpha, beta-ethylenically unsaturated dicarboxylic acid with a polyol, (2) 25 to 75 parts by weight of a vinyl monomer copolymerizable with said unsaturated polyester, (3) 1 to 25 parts by weight of a thermoplastic polymer, and (4) at least one member selected from the group consisting of hydroxyl-containing vinyl monomers and epoxy-containing vinyl monomers in a ratio of 1 to 40 parts by weight to 100 parts by weight of said unsaturated polyester (to form 100 parts by weight of the composition). The composition may be molded and cured to yield articles which provide good adhesion to surface coatings and are susceptible of decorative finishes.
Epoxy-containing vinyl monomers suitable for use in Ichimura et al. are described in column 4, lines 34-41. The monomers include the reaction products of alpha, beta-ethylenically unsaturated carboxylic acids and epichlorohydrin, and monoesterification products of diepoxy compounds with the unsaturated carboxylic acids. Preferred monomers include glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, and methallyl glycidyl ether. Ichimura et al. discloses only epoxy-containing vinyl monomers having reactive unsaturation for use in the polyester molding compositions therein.
U.S. patent application Ser. No. 194,069, filed in the name of Kenneth E. Atkins on Oct. 16, 1980 and titled "Improved Polyester Moldings", described a polyester molding composition comprising:
(a) a polyester resin comprising the reaction product of an olefinically unsaturated dicarboxylic acid or anhydride and a polyol;
(b) an olefinically unsaturated monomer that is copolymerizable with said polyester resin;
(c) a thermoplastic polymer low profile additive to control shrinkage; and
(d) an effective amount of a second crosslinkable vinyl monomer having a reactivity ratio (r.sub.1) with styrene of greater than 1. The addition of component (d) in said U.S. patent application Ser. No. 194,069 is stated to result in moldings with improved surface characteristics.